1. Field of the Invention
The present invention relates to a method of manufacturing an X-ray mask which is capable of making stress of an X-ray mask for use in X-ray lithography to be zero throughout the X-ray mask so as to pattern the X-ray mask with a required positional accuracy, and to a heating apparatus for heating the X-ray mask.
2. Related Background Art
FIG. 33 is a cross sectional view showing steps of a conventional method of manufacturing an X-ray mask disclosed in, for example, Japanese Patent Application No. 5-138104. Referring to FIG. 33, reference numeral 1 represents a silicon substrate, 2 represents a membrane (synonym with X-ray permeable substrate) formed on the silicon substrate 1, having a thickness of 1 .mu.m to 2 .mu.m and made of light elements, 3 represents a reflection preventive film formed on the membrane 2 and made of, for example, an indium tin oxide, 4 represents an X-ray absorber formed on the reflection preventive film 3 and made of a tungsten-titanium film, 5 represents a resist and 6 represents a support ring having a bonding adhesive 7 for bonding and supporting the silicon substrate 1.
A method of manufacturing an X-ray mask having the foregoing structure will now be described. Initially, the membrane 2 is formed on the silicon substrate 1 see FIG. 33A!. Then, a portion of the silicon substrate 1 is removed (etched back) see FIG. 33B!. Then, the reflection preventive film 3 is applied to the membrane 2, followed by being baked so as to be formed into a film see FIG. 33C!. Then, the X-ray absorber 4 is formed on the reflection preventive film 3 by, for example, a sputtering method. The mean membrane stress of the thus-formed X-ray absorber 4 is measured to determine temperatures with which the mean stress can be made to be zero. Then, annealing is performed uniformly in, for example, an oven, at 250.degree. C. to adjust the mean thin film stress of the X-ray absorber 4 to be zero see FIG. 33D!.
Then, the resist 5 is applied, and then baking is performed at, for example, 180.degree. C. see FIG. 33E!. Then, the silicon substrate 1 is bonded to the support ring 6 with the bonding adhesive 7 see FIG. 33F!. Then, electron beam drawing or development is performed to pattern the resist 5. The patterned resist 5 is used as a mask when the X-ray absorber 4 is dry-etched so that the X-ray absorber 4 is patterned. Then, the resist 5 is removed so that the X-ray mask is formed see FIG. 33G!. Note that the process for etching back the silicon substrate 1 and the process for bonding the silicon substrate 1 to the support ring 6 are not always performed in the foregoing sequential order.
The conventional method has been arranged such that the mean thin film stress of the X-ray absorber 4 is adjusted to be zero by changing the annealing temperature after the X-ray absorber 4 has been formed. A problem occurring when the film stress is not adjusted as described above will now be described with reference to FIG. 34. An assumption as shown in FIG. 34A is performed that the size of the window region of the silicon substrate 1 is 30 mm.times.30 mm and the stress of the X-ray absorber 4 at a position corresponding to the window region of the silicon substrate 1 is 10 MPa because no adjustment has been performed, as shown in FIG. 34B. If a 15 nm.times.15 nm region of the X-ray absorber 4 is patterned relative to the center of the window region of the silicon substrate 1 as shown in FIG. 34A, the stress of 10 MPa of the X-ray absorber 4 results in patterning of the X-ray absorber 4 being shifted for 15 nm from the center of the membrane 2. Thus, there arises a problem in that the X-ray absorber 4 cannot be patterned as required. The shift takes place outward when the stress is tensile stress, while the same takes place inward when the stress is compressive stress.
Accordingly, the X-ray absorber 4 has been heated by annealing to make the mean thin film stress of the X-ray absorber 4 to be zero so that unintentional shift occurring in patterning has been prevented.
The conventional method of manufacturing an X-ray mask has been performed as described above to prevent the patterning shift of the X-ray absorber 4 of a type shown in FIG. 34. However, since the actual distribution of the thin film stresses of the X-ray absorber 4 is not uniform, a problem as shown in FIG. 35 arises.
FIG. 35A shows a state where a 15 nm.times.15 nm region of the X-ray absorber 4 in a 30 mm.times.30 mm window region of the silicon substrate 1 has been patterned similarly to FIG. 34A. FIG. 35B is a graph showing stress distribution over the X-ray absorber 4 realized after the X-ray absorber 4 has been heated by annealing and, thus, the mean thin film stress of the X-ray absorber 4 has been made to be zero. Although the mean film stress of the X-ray absorber 4 has been made to be zero as shown in FIG. 35B, stress has not been zero throughout the X-ray absorber 4 in actual. Therefore, patterning of the X-ray absorber 4 shown in FIG. 35A is shifted similarly to the case shown in FIG. 34. Thus, there arises a problem in that the X-ray absorber 4 cannot be patterned as required.
In case of the X-ray absorber as shown in FIG. 35B, although the mean stress is equal to 0, the stress distribution in the thickness direction changes from -10 MPa to +10 MPa, so that the X-ray absorber has various stress not equal to zero at many points in the thickness direction. Therefore, when an overetching process is applied to where the mean stress is equal to zero, the mean stress moves to the compress stress side and becomes totally several MPa. Therefore, patterning of the X-ray absorber 4 shown in FIG. 35A is shifted similarly to the case shown in FIG. 34. Thus, there arises a problem in that the X-ray absorber 4 cannot be patterned as required.